Preview 6 (f5cbe67)

fix: fixes from an app-wide review

An app-wide review with several agents, then a second review of the
fixes. Among the changes:

• Ownership, writes, sizes and colours wait until the connection has
  attached the session, so Rex does not refuse them after a reconnect.
• The data socket reads with a 16 MiB budget, so a slow restore does
  not stop the pongs that keep it open.
• Keys left from a paste go out before keys typed after a Retry.
• A Rex restart drops the pane facts Rax kept, and waiting
  notifications go away for sessions Rax cannot attach.
• The fallback screen checks that the pane kept its size and times its
  drawing from a measured round trip.
• Selection handles use the whole selection, also off screen, and a
  dragged handle scrolls past the bottom edge.
• The emulators send programs no replies, as Rex's terminal answers
  them. Links open for http, https and mailto only. Pinch zoom sets the
  size when the fingers lift. Copies over 256 KiB are refused.
• The tailnet node stops 60 s after nothing needs it.
• Native fixes in the JNI bridge, snapshot text and Kitty image limits.
• Tests for the protocol client, the stream, the parser, the screen
  read, permissions and the terminal library.